iommu/arm: fix ARM_SMMU_QCOM compilation

My previous bugfix ended up making things worse for the QCOM IOMMU
driver when it forgot to add the Kconfig symbol that is getting used to
control the compilation of the SMMU implementation specific code
for Qualcomm.

Fixes: 424953cf ("qcom_scm: hide Kconfig symbol")
Reported-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reported-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reported-by: John Stultz <john.stultz@linaro.org>
Link: https://lore.kernel.org/lkml/20211010023350.978638-1-dmitry.baryshkov@linaro.org/


Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>